1. Field of the Invention
The present invention relates to an exhaust control device provided in an exhaust port opening on an inner peripheral surface of a cylinder hole of a two-cycle engine.
2. Description of Background Art
In a two-cycle engine an exhaust port and a scavenging port formed in an inner peripheral surface of a chlinder hole are opened and closed by a piston. Fresh air is compressed within a crank chamber and is fed from the scavenging port into a cylinder chamber. Exhaust gas is discharged out of the exhaust port. A "blow-by" occurs wherein during an up stroke of the piston, a portion of the fresh air is discharged out of the exhaust port during a period from the closure of the scavenging port to the closure of the exhaust port. A procedure has been heretofore carried out in order to reduce the blow-by of fresh air such that an exhaust control valve is provided in an exhaust port so that when the engine is operated with a low load, the upper portion of the exhaust port is closed, and when the engine is operated with medium and high loads, the exhaust port is fully opened to change the timing of the opening and closing of the exhaust port.
In such an exhaust control valve as described, Japanese Utility Model Publication No. 54336/81 discloses a rotary control valve in which a body portion is formed to be a substantially drum shape in the form of a recessed arc along the inner peripheral wall of the cylinder, and the surface of said body portion is positioned as close as possible to an exhaust port. The body surface of the control valve substantially functions as an upper edge of the exhaust port, and the body portion is scooped out so as to have substantially the same shape as the wall surface of the exhaust passage when the control valve is in a fully opened position.
Japanese Patent Application Laid-Open Publication No. 23523/87 discloses an exhaust control valve in which said valve is provided above an exhaust passage extending outwardly from an exhaust port opening to an inner surface of a cylinder, and a control surface having substantially the same curvature as the inner surface of the cylinder and matching to the inner surface of the cylinder is provided at the swinging end of the valve body supported for swinging by a shaft disposed perpendicularly intersecting to the cylinder axis. In the upper portion of the exhaust passage is provided a recess in which the valve body is received when the exhaust port is fully opened.
Further, Japanese Patent Application Laid-Open Publication No. 33426/91 also discloses an exhaust control valve in which similarly to the former, a control surface (a flow shutoff cover) is provided at the swinging end of a valve body (main body) supported for swinging within an exhaust passage. In Japanese Publication No. 33426/91, the control surface is in the form of a flexible sheet, which is received into a slot-like cavity provided in the upper wall of the exhaust passage.
In FIGS. 7 and 8 of the aforementioned publication, there is shown an arrangement wherein a housing is provided within an exhaust passage, and said slot-like cavity is formed by a clearance between the outer surface of the upper wall of the housing and the inner surface of the upper wall of the exhaust passage. The housing is provided within the exhaust passage with both side walls thereof placed in contact with both side walls of the exhaust passage. The valve body for swinging the control surface is disposed internally of the exhaust passage along the inner surface of the housing side wall.